Chemical processes on the industrial scale are available for production of chemical intermediates and end products. The processes can be operated batchwise, semicontinuously, continuously or in a combination of one of the three variants. The processes are endothermic or exothermic and can be conducted isothermally or adiabatically. According to the chemical product, the process can be conducted in the gas phase or liquid phase, with or without solvent, or in the melt. The workup and purification of the chemical product thus obtained can be effected by one of the standard methods in the art, for example crystallization, washing or distillation, or in a combination of these workup methods.
Chemical intermediates and end products are, for example, (poly)isocyanates and precursors thereof, polycarbonates and precursors thereof, active pharmaceutical ingredients and precursors thereof, or active ingredients for crop protection and precursors thereof.
The quality of a process for preparing chemical products is firstly defined by the content of unwanted by-products in the product of the process. Secondly, the quality of a process is defined in that the whole operation of startup and normal production until the operation is run down can be executed without technical production outage or problems that necessitate intervention in the operation, and that there are no losses of feedstocks, intermediates or end product.
Ideally, therefore, the industrial scale plants for performance of such preparation processes are designed such that the processes run in a robust manner in the event of appropriate quality of the auxiliaries and feedstocks used and correct choice of process parameters such as pressure, temperature, ratios of amount and concentrations of the auxiliaries and feedstocks, etc. This means that, in such continuously operated large-scale plants, there will ideally be no problems such as the formation of precipitates, which can settle out in plant equipment or block pipelines.
The startup and shutdown of continuously operated large-scale chemical plants poses particular challenges for the person skilled in the art, caused by the problems that can occur spontaneously therein. Such problems include, for example, cessation of the reaction, elevated energy demand to start the reaction, incomplete reaction which can lead to overloading of the workup, formation of elevated amounts of unwanted by-products which can lead to quality and/or safety problems, deactivation, damage to or carbonization of catalysts, formation of deposits which can block the equipment, or caking of product, by-products and/or substrates.
Production shutdowns that necessitate the shutdown and startup of the plant are an everyday occurrence in industry. Such a shutdown may be an inspection shutdown which is planned in advance, for which purpose the plant is run down, the energy sources are switched off and typically all plant sections that are to be inspected are opened and cleaned for the purpose of examination. Such an inspection may take one or more weeks. After the inspection has ended, the production plant is closed, optionally inertized and provided with auxiliaries and, once the appropriate energy sources and raw materials are available, the production plant can be started up again. However, a production shutdown is not necessarily associated with opening or another mechanical intervention into a reactor or another apparatus in the plant, but may also be connected to the shutdown and restart of the production plant for various other reasons, for example in the event of outage of the raw material supply. In such a case, the plant is typically run in part-load operation and, in the worst case, when the logistical supply chain is interrupted, has to be shut down. Furthermore, production shutdowns may be forced by requirements for maintenance, cleaning or repair in the production plant. In the nitrobenzene process, for example, shutdowns are described as short when production is interrupted for up to one day. It is a feature of all these production shutdowns in practice that there are losses of production, and that, on restarting of the plant, for example when inertization is necessary, nitrogen is consumed or, in the heating of the plant or the feedstocks, energy has to be supplied in the form of steam and power.
The person skilled in the art is aware that an industrial process operated semicontinuously or continuously proceeding from a production plant in operation cannot be switched instantaneously to a production shutdown, but has to be run down in a controlled manner beforehand. This is also the case for a plant stoppage in the event of an emergency. In order to be able to produce again after a production shutdown, the plant has to be run back up to the process parameters before the production shutdown. Reactants and apparatuses have to be heated up, apparatuses may have to be inertized, and the loading of the apparatuses with the reactants is gradually increased to the desired target value. During this startup phase, there is still loss of production, and a disproportionate amount of energy has to be expended in order to prepare the cooled plant for startup and then to run it up to the desired target value with observation of all operationally relevant parameters as well.
A further disadvantage is that, in the event that maintenance, repair and cleaning operations are necessary on or in a reactor or another plant section, it is regularly necessary always to switch off all plant sections since the process steps build on one another and hence always proceed successively. As a result, it is necessary to empty the entire plant, which leads to a considerable amount of reject material. Furthermore, energy has to be expended in order to bring reactors and plant sections back to the respective operating temperatures. Such production shutdowns for plant inspections, repair, maintenance and cleaning measures or shortfalls of raw material or auxiliary that occur, whether planned or unplanned, are therefore recurrent plant states which have a considerable influence on the economic operation of a plant or process that works continuously.
The current prior art processes for industrial scale preparation of chemical products do generally succeed in preparing the desired products with high yield without significant loss of quality in the respective end products, but the literature describes almost exclusively processes in normal operation.
Only for a few industrial scale processes are there descriptions relating to avoidance of problems during the startup and shutdown phase of these processes. For instance, WO 2014/016292 A1 describes the startup of the process for nitrobenzene preparation, wherein the starting material used is a benzene containing less than 1.5% aliphatics, one effect of which is that an improved reaction rate of the reaction is ensured, which saves steam and minimizes the formation of the picric acid by-product. Similar positive effects are described by WO 2014/016290 A1 in relation to the startup procedure for the nitration of benzene to nitrobenzene, wherein circulating sulfuric acid containing less than 1.0% nitrobenzene is used, one effect of which is to save steam, since the workup is not burdened by additional benzene that occurs through incomplete nitration, and the formation of the dinitrobenzene and picric acid by-products is minimized.
The industrial scale continuous preparation of aniline by hydrogenation of nitrobenzene is described in WO 2013/156410 A1, wherein specific reference is made to the startup of the hydrogenation process after regeneration of the hydrogenation catalyst. This involves undertaking an exchange of water in a liquid ring compressor, in order to separately dispose of carbon dioxide present therein, which avoids blockages and corrosion in the waste air system by ammonium carbonate. WO 2013/156409 A1 likewise describes the preparation of aniline, with attention given to the shutdown of the hydrogenation of nitrobenzene, the inventive procedure in the shutdown of the reaction involving the presence of hydrogen for long enough in a sufficient amount in the reaction spaces for all nitroaromatics to have reacted with hydrogen and hence complete depletion thereof during the shutdown operation of the hydrogenation. This prevents increased by-product formation and carbonization of the catalyst by nitrobenzene residues which are hydrogenated with too small an excess of hydrogen. Furthermore, the cost and inconvenience associated with catalyst regeneration is minimized and the service life of the reactors is prolonged.
Patent application WO 2006/125759 A1 is concerned with a Fischer-Tropsch production plant and a process for preparing hydrocarbons. Matters that the patent application addresses are how production operation can still be maintained at least to a restricted degree for a limited time in the event of disrupted availability of methane, and how the production plant can be run down in a controlled manner. One way in which this is achieved is by provision of a reservoir tank 20 for liquefied natural gas. It is mentioned that it can be advantageous, for example, in the event of emergency switch-off of the gasifying unit, to keep the unit at relatively high temperature in order to be able to accomplish the later startup more quickly (called “hot standby”). Also described is an embodiment in which a distillation unit 18 is connected downstream of a hydrocracking reactor 16 and a partial stream of the output from the distillation unit 18 can be recycled into the hydrocracking reactor, while the remaining output from the distillation unit 18 is sent to its normal end use (cf. FIG. 1). No general teaching relating to circulation operation of individual plant sections during the simultaneous complete shutdown of other plant sections in such a way that maintenance operations, for example, can be conducted in these other plant sections can be inferred from this patent application.
Patent application US 2008/026112 A1 is likewise concerned with Fischer-Tropsch processes, specifically with the case of temporary interruption of production in a triphasic reaction zone comprising a liquid phase, a gas phase and a phase comprising suspended catalyst particles. What is disclosed is interruption of the reactant supply and inertization of the reaction zone with an inhibiting gas. An embodiment is described in which the inhibiting gas is recycled into the lower portion of the reaction zone in order to keep the catalyst particles in suspension. In general, the reaction is followed by a condensation step and a separation step, in which case the gas phase from the separation step is recycled into the lower portion of the reactor. To restart the reaction, the reaction zone is charged with an activating gas. No general teaching relating to circulation operation of individual plant sections during the simultaneous complete shutdown of other plant sections in such a way that maintenance operations, for example, can be conducted in these other plant sections can be inferred from this patent application.
Patent application WO 2013/053371 A1 is concerned with a method of providing a methane-rich product gas and an arrangement suitable for the purpose. In the event of a lack of hydrogen, the reactor is operated on standby (without further continuous reactant gas supply). At the changeover between normal operation of the reactor and the standby state, there is intermittent formation of a product gas that does not meet the usual quality demands (also referred to as “inferior gas”). This inferior gas, after restarting of normal operation, is at least partly recycled to a process step that precedes the methanization. This is preferably accomplished by gradually feeding in intermediately stored inferior gas with a time delay. With regard to the standby operation itself, the patent application gives barely any information. It is disclosed merely that the reactor can be purged, for example with hydrogen gas. No general teaching relating to circulation operation of individual plant sections during the simultaneous complete shutdown of other plant sections in such a way that maintenance operations, for example, can be conducted in these other plant sections can be inferred from this patent application.
Patent application US 2005/0227129 A1 is concerned with a heating apparatus for fuel cell power plants. The fuel cell power plant comprises—cf. FIG. 1 and the accompanying text passages—three reactors, namely a reformer (3), a carbon monoxide converter (4) for reaction of carbon monoxide with steam, and a further converter (5) for reaction of carbon monoxide with oxygen. Chemical reactions take place in all these reactors. Without retrofitting measures, they are unsuitable for working up and purifying the process product of a chemical reaction with removal of secondary streams. The heating apparatus according to US 2005/0227129 A1 comprises a burner (6) and conduits (71 to 73) for the supply of hot combustion gases for the purpose of heating the fuel cell power plant. Controlled recycling of the starting stream from an apparatus through these conduits as input stream into this or an upstream apparatus is obviously not envisaged, and does not appear possible either without further retrofitting measures such as the incorporation of additional valves. Thus, no general teaching relating to circulation operation of individual plant sections during the simultaneous complete shutdown of other plant sections in such a way that maintenance operations, for example, can be conducted in these other plant sections can be inferred from this patent application either.
What would be desirable would thus be processes and plants for preparation of chemical products where it is possible to optimize production shutdowns in the course of operation of the respective operations in terms of time taken and possibly also with regard to energy and material consumption. These would lead to a not inconsiderable improvement in the productivity and hence the economic viability of industrial scale chemical production operations and the corresponding plants.